The present invention relates to polyisocyanates containing 4,4'-diisocyanato dicyclohexylmethane having a high content of the trans,trans isomer. More specifically, the present invention relates to a free flowing solid polyisocyanate based on 4,4'-diisocyanato dicyclohexylmethane.
4,4'-diisocyanato dicyclohexylmethane (alternately referred to as "PICM") is a cycloaliphatic diisocyanate of low volatility. PICM and other aliphatic isocyanates are useful in the preparation of non-discoloring polyurethanes. In general, such isocyanates are reacted with glycols and/or polyols and chain extenders and/or cross linkers. Such isocyanates are particularly useful in the preparation of polyurethane coatings and elastomers. PICM and the diamine precursor, 4,4'-diamino dicyclohexylmethane ("PACM"), exist, in three stereoisomeric forms (i.e., trans,trans; cis,trans; and cis,cis) as described, for example, in U.S. Pat. Nos. 2,606,925, and 3,789,032, Canadian Patents 961,049 and 971,184, and British Patent 1,220,715. Commercial grades of PACM and PICM normally contain all three isomers.
The most direct method of producing PICM is to first hydrogenate diamino diphenylmethane to form a mixture of the stereoisomers of PACM, and to then phosgenate the mixture. When the synthesis of PICM is conducted using readily available mixtures of stereoisomers of PACM (such as the equilibrium mixture described in U.S. Pat. No. 3,155,724), the PICM obtained is a slush or slurry at normal operating temperatures, having a melting point of about 58.degree. C., which corresponds to a trans,trans-isomer content of about 54%. Various art-known PICM mixtures have trans,trans-isomer contents of from about 17 to about 55% by weight. In addition, the art has recognized an advantage in utilizing high trans,trans-isomer PICM in producing elastomers (see, U.S. Pat. No. 3,789,032).
In order to prepare PICM of relatively high trans, trans-isomer content, the art has generally used a PACM having a relatively high trans,trans-isomer content in the phosgenation reaction. Various methods are known for treating PACM to obtain the requisite high trans,trans-isomer content. Crystallization techniques have been described in the art. See, e.g., U.S. Pat. Nos. 2,494,563, 3,153,088, 3,384,661 and 3,393,236. The crystallization of PACM suffers from various disadvantages. PACM readily forms a precipitant when exposed to carbon dioxide, causing problems in filtering and contamination of the crystals (see, U.S. Pat. No. 2,494,563, column 3, lines 26-29, and column 4, lines 72-75). In addition, PACM is generally difficult to crystallize since it will easily form a supercooled liquid. The prior art has overcome this problem by adding seed crystals (U.S. Pat. No. 2,494,563), by lowering the viscosity by using an inert solvent (U.S. Pat. Nos. 2,494,563, 3,153,088, 3,393,236 and 3,384,661), or by forming an adduct of PACM that crystallizes better, such as the hydrate (U.S. Pat. No. 3,153,088) or the alcoholate (U.S. Pat. No. 3,384,661 ). Such an adduct must be treated to remove water or alcohol before phosgenating to PICM.
U.S. Pat. No. 4,983,763 describes a process for preparing PICM of relatively high trans,trans isomer content; said process comprises crystallization of PICM. However, it does not teach that PICM can be obtained in the form of a free-flowing solid.
Generally put, it seems that although processes for preparing PICM of high trans,trans contents are known in the art, the nature of PICM as free-flowing solids, in commercially useful quantities, has not been realized. To date, the art known commercially available PICMs are obtained in the form of solutions and slurries. The present invention provides in commercial quantities a free-flowing solid based on 4,4'-diisocyanato dicyclohexylmethane.